Bio 1000- DNA organization

DNA ORGANIZATION

In eukaryotes, there are two major proteins: histone proteins and non-histone proteins, which is associated with DNA structure and regulation. These proteins are generalized as chromosomal proteins. The complex of DNA and its associated proteins, termed chromatin is the structural building block of a chromosome.

HISTONE PROTEINS

HISTONES are class of small, positively charged proteins that are complexed with DNA in chromosomes of eukaryotes. The histones link to DNA by an attraction between their positive charges and negatively charged phosphate groups of the DNA.

Five types of histones exist in eukaryotic cells : H1, H2A, H2B, H3 & H4. FUNCTION: To pack DNA molecules into narrow confines of the cell nucleus.

HISTONES & DNA PACKING

Histones pack DNA at several levels of chromatin structure. Most fundamental structure is the nucleosome. HOW DOES IT WORK?

1. Two molecules, each of H2A, H2B , H3 & H4 combine to form a beadlike, eight protein nucleosome core particle around which DNA winds for almost 2 turns
2. Short segment of DNA, LINKER, extends one nucleosome and the other

The fifth histone H1 brings the next level of chromatin packaging.

1. One H1 molecule binds both the nucleosomes and the linker DNA
2. Binding causes the nucleosomes to package in coiled structure called the SOLENOID

This arrangement of DNA compacts the DNA and protects it from chemical and mechanical damage.

HETEROCHROMATIN VS. EUCHROMATIN: WHAT THE HECK IS THAT?

 

During interphase nuclei, chromatin fibres are loosely packed in some regions and densely packed in other regions. Loosely packed regions are known as EUCHROMATIN. Densely packed regions are called HETEROCHROMATIN.

 



 NON HISTONE PROTEINS

Non-histone proteins are loosely defined as all the proteins associated with DNA that are not histones. Most are negatively charged or neutral but exceptions are for those that are positively charged. Many non histone proteins help control the expression of individual genes. Many non histone proteins also affect gene accessibility by modifying histones to change how they associate with DNA in chromatin, either loosening or tightening the association. Note, in euchromatin, the gene is more accessible and in heterochromatin, the gene is less accessible.